1. Field of the Invention
The present invention relates to a method of manufacturing packaging containers and more particularly to a method of manufacturing parallelepiped packaging containers by positioning the vertically sealed portions of the parallelepiped containers in the corners of the side walls.
2. Prior Art
In the prior art, methods of manufacturing brick-shaped packaging containers filled with liquid foodstuffs such as milk and juice have used a tape-shaped packaging web provided with folding lines and a plastic coating. In the filling section of the packaging machine such a web is shaped into a tubular configuration, filled with the contents, sealed and then cut separately at predetermined intervals. The top and bottom parts of the packaging container blank thus obtained are folded in order to obtain individual parallelepiped-shaped packaging containers.
In order to create the above described tubular configuration, a packaging material web moving laterally on the upper part of the packaging machine is bent down nearly at right angles and the resulting bent web surface is transferred downwards and converted by bending guides into a tubular configuration. The longitudinal edges are heat-sealed together to form a tube with longitudinal liquid-tight sealed spaces which are then filled with the liquid contents. The liquid-tight sealing portion is in the middle of the side surface of the tube. The tube is then filled with a liquid and is shaped into a square crosssection by the molding jaws of a molding and sealing device, which successively opens and closes along the lateral direction of the moving packaging container blanks. As a result, the longitudinal sealing portion zones are formed in the middle of one of the side walls (normally the wider side).
In Japanese Laid-Open Patent Publication No. 56-95807 a device is disclosed wherein the entire lateral sealing device is attached to a turnable base plate for sealing the material tube at predetermined intervals. This base plate is turned to predetermined positions for lateral sealing and produces separated packaging container blanks with longitudinal sealed portions between neighboring side walls.
In existing manufacturing methods for packaging containers it is unavoidable for longitudinal sealed portions to be located in the middle of the side wall along the lateral transferring direction of the packaging container blanks at the point where the material tube is shaped into a rectangle in cross-section under the action of the sealing jaws which open and close in association with the movement of the material web or the above described packaging container blanks (see FIG. 12). Consequently, a part of the printed patterns on the material web is not visible at the middle portion of one of the side walls. Therefore, parallelepiped packaging containers manufactured as described above suffer from the printed patterns abutting in the middle of one of the side walls (usually the wider side), resulting in an unsatisfactory appearance due to the staggering of the printed pattern at the sealed portion. In addition, the mechanical strength at the corner portions of this type of packaging container is reduced due to lack of a sealing joint, resulting in poor dimensional stability.
Another device disclosed in Japanese Laid-Open Patent Publication No. 56-95807, discloses forming longitudinal sealed portions at the side wall corners of the packaging containers. However, practical application of this disclosure is very difficult due to the complicated construction of the entire lateral sealing device which must be turned to a predetermined position for sealing the material tubes at predetermined intervals.